The field of the invention is mixtures of thermoplastic synthetic resins with additives and cyclic esters.
The state of the art of cyclic esters useful in the present invention may be ascertained by reference to U.S. Pat. No. 3,021,313, the disclosure of which is incorporated herein and the state of the art of compounding and processing of thermoplastic synthetic resins may be ascertained by reference to the Kirk-Othmer "Encyclopedia of Chemical Technology", 2nd Edition (1968), vol. 15, pp. 800-807, the disclosures of which are incorporated herein.
As disclosed in Kirk-Othmer, the compounding of plastics may be done on a two-roll mill, an extruder, or a Banbury (Farrel Corp.) mixer. Concentrates or master batches may be produced and blended with virgin resin on a mill before final processing. The rolls on a two-roll mill operate at different speeds and temperatures. In spite of the simplicity of the milling operation, extrusion is usually preferred since an extruder can operate continuously to produce strands that may be cut by a rotating knife to produce uniform pellets.
A conventional process for the introduction of additives into thermoplastic synthetic resins resides in that the additives are added in a pug mill to the finished polymers or polycondensates and mixed in the melt, for example in an extruder. Apart from the fact that this operation represents an additional working step, this method of incorporation is unsuitable for distributing finely divided additives uniformly in the synthetic resin. This holds true, in particular, if minor amounts of optical brighteners, stabilizers, flatting agents, or similar materials are to be homogeneously distributed in a synthetic resin.
It is furthermore known to disperse the additive in one of the starting materials required for the production of the synthetic resin, for example in ethylene glycol [starting compound for poly(ethylene terephthalate)], and to add this dispersion during the manufacture of the polymer. The primary disadvantage in this process is that the additive can decompose under the relatively vigorous reaction conditions which almost always are ambient in the manufacture of synthetic resins, triggering discolorations and degradation reactions. Besides, an agglomeration of the additive occurs frequently under the reaction conditions, leading to an inhomogeneous distribution of the additive in the synthetic resin.
The danger of thermal degradation of the additive can be reduced by utilizing a premix wherein the additive is dispersed in a concentrate (masterbatch), corresponding to the actual synthetic resin or compatible with the latter. This premix is generally added at the end of the synthetic resin manufacturing process. Such suggestions disregard the fact that the same above-mentioned problems can occur during the preparation of the premixes.
Although the agglomeration of the additive in the monomer dispersions can be diminished by auxiliary dispersing agents which are to maintain the dispersion in stable condition even under the manufacturing conditions of the synthetic resin, these auxiliary agents, apart from the expense of obtaining them, can impair the properties of the synthetic resin in an undesirable manner.